B-12 Injectables

I tried searching for this but couldn't find any answers. I am looking into getting some b-12. Does this always have to be prescribed? I know of some websites that carry it. But was wondering what the laws were or if anyone knew where I can read the laws. Here in Las Vegas there are plenty of horse supply stores so I'm sure I can by it there, if it is legal? Thanks for the help bro's.